The present invention relates to a device or machine for subjecting a test object to an acceleration pulse of a prescribed shape and amplitude. Although initially intended for testing crash sensors used as part of an automobile air bag restraint system, the machine has more general capabilities, as will be described below.
In order to verify that crash sensors have the proper response, all crash sensors are tested during the final manufacturing process and data relative to this test is stored. The sensor testing process involves subjecting the sensor to a series of simulated crash pulses which are typically idealized as either half sine or haversine shaped acceleration time pulses. Typically, a sensor is tested with a series of six such pulses. The ball-in-tube sensors used in the crush zone for Ford and General Motors cars, for example, are all tested with haversine pulses having durations of 10, 20 and 30 milliseconds. For each duration, the sensor is subjected to a pulse where it should not trigger and a second pulse where it must trigger by a certain time period. If a sensor passes all six tests, it will meet the inspection requirements set forth and can be shipped and installed on a vehicle. If it fails any one of the six tests, the sensor will fail inspection.
A variety of devices have been used to create the pulses used for these tests. One system, for example, uses a cam consisting of a disk approximately six feet in diameter which is rotated at a constant speed. The sensor is mounted onto a cam follower and each of the test pulses is machined into the cam surface. As the cam makes one revolution, the sensor is subjected to the required series of pulses.
This cam system has largely been replaced by an electromagnetic linear actuator, referred to commercially as a "shock pulse system" which has since become the standard device for testing sensors. A typical actuator consists of a voice coil (or acoustical coil) and electromagnet combination. The device is typically approximately two feet in diameter and the voice coil is typically about six inches in diameter and about six inches long. Flux lines from the electromagnet are concentrated in a gap through which the voice coil travels. When current flows in the voice coil, a force is exerted on the voice coil which, in turn, propels an armature in a linear direction. The armature is typically about two feet long and a sensor is mounted on the opposite end of the armature from the voice coil. A computer generates the required pulse which is fed to an amplifier that creates the necessary current for driving the voice coil. The entire voice coil and armature combination weighs typically about ten to fifteen pounds. In order to create the required test pulses as much as 300 amperes of current at up to 300 volts must be supplied to the voice coil.
This current is generated by the amplifier which is normally of the switching type. The actuator device typically weighs about 2,000 pounds. In addition, ancillary equipment is required to cool the device to prevent overheating. Testing devices of a design similar to that described above can be purchased commercially at a price in excess of $100,000.
Test equipment such as the actuators described above are a required component in the manufacturing of ball-in-tube crash sensors such as disclosed in Breed U.S. Pat. No. 4,329,549. This equipment has been reported to be the least reliable device on the manufacturing line. In particular, the pulses created by the actuating devices have been found to vary between individual devices and over time, and are a function of temperature. Thus, sensors which pass a test on one actuating device might fail the test on the same device at a different time or on a different device.
Crash sensor test device problems have many causes, but primarily are related to the fact that a test device does not inherently produce a perfect haversine or half sine pulse. The computer specifies a pulse to the amplifier which creates the current which drives the voice coil at one end of the armature. Accelerometers at the other end measure the resulting pulse and the computer compares this pulse with the desired pulse and modifies or corrects the pulse that it sends to the amplifier. This process is repeated until the accelerometer pulse is judged satisfactory. Usually substantial filtering of the output pulse is required before a comparison can be made. This is due to the fact that there are many factors which influence the signal received from the accelerometer which are random in nature and therefore cannot be corrected by modification of the computer generated pulse.
The problem begins with the fact that accelerometers themselves are typically only accurate to approximately 1%. The signal from the accelerometers passes through a thin wire which must travel with the accelerometer as it moves approximately 12 inches during each pulse. Although care is taken to shield the accelerometer wires from stray electromagnetic fields, this has been only partially successful. In particular, the switching amplifiers used to supply current to the voice coil are noisy and radiate a large amount of energy at the switching frequency. The voice coil itself acts as an antenna radiating high frequency oscillations from a point inches away from the accelerometers.
The voice coil provides a force at one end of a long armature. The sensor to be tested and the accelerometers are at the other end of this armature. The armature has many natural frequencies, one of which is typically around 1,000 Hertz. Thus, there is a time delay between the time that current flows through the voice coil and accelerates one end of the armature and the time that the accelerometers sense the acceleration at the other end. Any acceleration to one end of the armature excites natural frequencies in the armature which are superimposed upon the desired pulse. In addition the armature typically is supported by rolling element bearings which also excite vibrations in the armature. This is caused by the rollers themselves and is compounded by roughness on the surfaces on which the bearings roll. This, in turn, can be caused by dirt which can attach onto these surfaces.
There are many other forces which effect the motion of the armature and which cause the actual pulse to deviate from the desired pulse. These include variations in the weight of the sensor being tested, the tension of the wires which carry the information from the moving sensor to the data acquisition system, and the copper bands which carry the current to the voice coil.
The natural frequency associated with the armature as mentioned above induces a significant lag between the time that the current flows to the voice coil and the time that acceleration is measured by the accelerometers. It is therefore not practical to use feedback to correct the pulse. This substantially exacerbates the problem since either wide tolerances on the acceptable pulse must be permitted or a feed forward correction must be made and the pulse rerun. This is particularly complicated in at least one brand of sensor tester where the correction process is done through a complicated mathematical procedure involving the use of fast Fourier transforms. For this particular actuating device, the force on the voice coil is dependent upon the location and the velocity of the voice coil, thus to achieve a certain acceleration, a different current is required depending on the exact position and motion of the voice coil. This particular testing device design, although capable of providing more accurate haversine acceleration pulses, requires as much as 40 seconds to run a series of six pulses. This requires either that multiple sensors be run simultaneously on the test device or that multiple test devices be used on a production line. In an alternate design of this test device, this cycle takes place in approximately five seconds, however, the accuracy and repeatability of the pulse is considerably compromised.
Test devices are designed to test sensors of widely varying requirements. For example, a standard crush zone sensor might require a 13 mile per hour test pulse with a duration of 10 milliseconds, resulting in a peak acceleration of approximately 100 G's, whereas a safing or arming sensor may require a 30 millisecond pulse of 1.3 mile per hour with a peak acceleration of about 2.5 G's. Test devices which have been designed to achieve the higher pulse have a great deal of difficulty accurately repeating the smaller pulse. The result has been that very wide manufacturing tolerances are required for arming sensors to compensate for the inability of current testing devices to test the sensor accurately.
In summary, devices currently being used to test crash sensors are relatively inaccurate primarily because of their inability to generate a clean, accurate acceleration pulse. They are large, heavy and expensive. These problems inherently stem from the use of a linear actuator as the fundamental actuating device and the use of an electromagnet to generate an acceleration pulse.
Another application of the present invention is in the testing of complete vehicle occupant restraint systems. With the rapidly increasing concern for automobile safety, there is a greater and greater need for devices to test various occupant restraint system designs. Full scale car crashes are expensive and as a result most testing in done on a sled which simulates a car crash using a portion of the car interior, referred to as a "vehicle buck" which is either accelerated or decelerated to achieve the desired crash pulse. The sled usually rides on rails and is quite large taking up to 140 linear feet of floor space. The cost of such sleds is also quite high and this is especially so for sleds which are capable of achieving the higher velocities which will be required as cars are designed for higher velocity impacts.